Conventionally, in an image forming apparatus employing an electrophotographic method, a charging device employing a corona discharge method has been used (i) as a charging device for charging a photoreceptor, (ii) as a transfer device for electrostatically transferring a toner image formed on a photoreceptor or the like onto recording paper or the like, (iii) as a removing device for removing recording paper or the like that has electrostatic contact with a photoreceptor or the like, and the like devices.
Such a charging device employing a corona discharge method includes a shield case having an opening facing a charge receiving material, such as a photoreceptor, recording paper, or the like, and a line or saw-like form discharge electrode to be provided in a tensioned state in the shield case. As such a charging device, for example, a so-called corotron for uniformly charging a charge receiving material by generating corona discharge by applying a high voltage to the discharge electrode, or a so-called scorotron for uniformly charging a charge receiving material by applying an intended voltage to a grid electrode provided between the discharge electrode and the charge receiving material (see, for example, Patent Literature 1) is used.
Patent Literatures 2 and 3, for example, disclose such a corona-discharge-type charging device used as a pretransfer charging device for charging a toner image to be transferred to a transfer medium, such as an intermediate transfer member or recording paper, before the toner image is transferred. Even if an amount of charge in the toner image formed on an image bearing member is uneven, the techniques disclosed in Patent Literatures 2 and 3 makes a charge amount of the toner image uniform before its transfer. As a result, it is possible to restrain a decrease in flexibility of a transfer when the toner image is transferred and to stably transfer the toner image onto the transfer medium.
However, such conventional charging devices have a lot of problems. Firstly, such a charging device needs not only a discharge electrode but also a shield case, a grid electrode, and the like. Further, it is necessary to ensure a predetermined distance (10 mm) between the discharge electrode and a charge receiving material. As a result, a lot of space for the charging device to be placed in is required. Generally, a developing device, a first transfer device, and the like devices are provided around a first transfer section, and a photoreceptor, a second transfer device, and the like devices are provided before a second transfer section. Therefore, there is only a small space for a pretransfer charging device to be placed in. For this reason, the conventional corona-discharge-type charging device causes a difficulty in a layout of such constituent devices in an image forming apparatus.
Secondly, the conventional corona-discharge-type charging devices have a problem that a great amount of discharge products such as ozone (O3) and nitrogen oxide (NOx) are generated. Generation of a great amount of ozone causes problems, such as generation of ozone smell, adverse effects on a human body, and deterioration of components due to strong oxidation. Further, generation of nitrogen oxide causes a problem that the nitrogen oxide is attached to a photoreceptor as ammonium salt (ammonium nitrate) and causes a defective image, and the like problems. Especially, an organic photoreceptor (OPC), which is generally used, is easily affected by ozone or NOx and causes image defects such as white spots and image deletions.
In order to prevent these problems, in an intermediate-transfer-type color image forming apparatus having a plurality of transfer members, it is preferable that a pretransfer charging device be provided in an upstream of all the transfer members (a plurality of first transfer members, and a second transfer member). This is preferable from the viewpoint that a charge amount of a toner image becomes uniform before the toner image is transferred. However, from a practical standpoint, the providing of a pretransfer charging device in such a manner is difficult due to a problem of a generation amount of ozone and NOx.
Further, in recent years, for the purpose of decreasing ozone, a contact charging method using a conductive roller, a conductive blush, or the like has been employed in a charging device for charging a photoreceptor itself. However, it is difficult to charge the photoreceptor without making a toner image uneven by such a contact charging method. Accordingly, a charging device employing a non-contact corona discharge method is used as a pretransfer charging device. However, in a case where the conventional corona-discharge-type pretransfer charging device is provided in an image forming apparatus including a contact charging device, an ozoneless effect is not exhibited.
As a technique to reduce a generation amount of ozone, for example, Patent Literature 4 discloses the following charging device. That is, a charging device in Patent Literature 4 includes: multiple discharge electrodes aligned at an approximately predetermined pitch along an axial direction; a high-voltage power supply for applying to the multiple discharge electrodes a voltage of not less than a discharge-initiating voltage; a resistor provided between an output electrode of the high-voltage power supply and the multiple discharge electrodes; a grid electrode provided, close to the multiple discharge electrodes, between the multiple discharge electrodes and a charge receiving material; and a grid power supply for applying a grid voltage to the grid electrode. In the charging device, the multiple discharge electrodes and the grid electrode are provided so that a gap therebetween is not more than 4 mm, thereby reducing a discharge current so that the generation amount of ozone is reduced.
However, although the technique disclosed in Patent Literature 4 can reduce the generation amount of ozone by reducing the discharge current as such, the reduction amount of ozone is still insufficient and around 1.0 ppm of ozone generates. Further, there has been another problem of unstable discharge caused due to attachment of discharge products, toner, paper dusts, or the like to the discharge electrodes, abrasion/deterioration of the heads of the discharge electrodes, or the like factor. Further, it is difficult to clean such discharge products, toner, paper dusts, or the like attached to the discharge electrodes because of the shape of the discharge electrodes.
Besides, a gap between the discharge electrodes and the charge receiving material is so narrow that unevenness in charge in a longitudinal direction (in a direction along the pitches between the discharge electrodes) is easily caused due to the pitches between the multiple discharge electrodes. There is another considerable method in which pitches between discharge electrodes are formed small to prevent such unevenness in charge. However, this increases production costs because the number of discharge electrodes is increased.
In view of the problems of the conventional charging devices, Patent Literature 5, for example, discloses a charging device including an ion generating element (a creeping discharge element) that includes: a discharge electrode having pointed protrusions in an outer circumference thereof and an induction electrode, the electrodes being provided so as to sandwich a dielectric member therebetween. The ion generating element generates ions by applying a high alternating voltage between these electrodes (hereinafter, this charge method is referred to as a creeping discharge method).
The creeping discharge type charging device is small because the device does not include a shield case, a grid electrode, and the like. Further, since a discharge surface is flat, the charging device is easily cleaned and excellent in maintenance.
A discharging characteristic of the ion generating element (the creeping discharge element) tends to decline under a high humidity condition. In view of this problem, for example, Patent Literatures 6 and 7 disclose techniques for improving the discharging characteristic by providing an ion generating element with a heater member and heating the ion generating element to remove absorption moisture in a discharge region. Especially, Patent Literature 7 discloses a technique in which an induction electrode generates Joule heat while a power is being supplied to the induction electrode, so that the induction electrode also works as a heater. With the technique disclosed in Patent Literature 7, it is possible to make the ion generating element small and to reduce costs, in comparison with a technique in which a heater element is separately provided.